When I melt 16 oz of chocolate I add 4 oz of oil how long can I keep this and how should it be stored. Someone said something about botulism ??
Botulism is a risk with unprocessed food which has been close to the soil, so fresh vegetables, or especially alliums (garlic!). It is not present in commercially produced chocolate - if it were, it would grow inside the bar. Remember, there is no oxygen in the middle of a chocolate bar either!
Normally, you can't expect that the mixture of two shelf-stable substances is also shelf stable, as you may add back the one factor which was keeping bacteria out. But in the case of chocolate mixed with an oil (or any other kind of pure fat, such as cocoa butter), this is not a problem.
Your mixture is shelf stable, and safe to consume for years. The only thing you have to care about is rancidity.